Coil bobbins, usually made of non-conducting material, support a winding or windings which are used in magnetic components such as solenoids, inductive chokes and transformers.
In high frequency transformers the number of turns required in a first section of a transformer, which will be referred to as the secondary winding, is often low—sometimes as few as 1-5 turns meet operational requirements. However the number of turns required in a second section of the transformer, which will be referred to as the primary winding, can be considerably higher, possibly around 100 turns, depending on the input/output voltages to be used and to be produced. Current in the primary winding is usually low.
Where only a small number of turns are required, planar components or lead frames can be used to achieve several important advantages.
Planar components for the present purpose are coils which have been manufactured by Printed Circuit Board (PCB) technology. Such components are often double sided. Various planar circuits are used, and reference is made to U.S. Pat. No. 5,010,314 by the present assignees for fill description thereof.
Lead Frames are coil spirals made by cutting or etching a thin copper sheet.
Both types of planar component, in comparison with conventional coils,    allow easy and fast assembly of a coil;    decrease skin and proximity effects;    demonstrate excellent thermal performance and can carry a heavy current;    facilitate good production repeatability and so have consistent properties; and    improve magnetic coupling and reduce the leakage inductance.
Examples of bobbin construction known in the prior art are seen in several U.S. patents. In U.S. Pat. No. 4,257,027 Yasuhisa discloses a transformer bobbin assembly. The device has a rectangular inner drum and an outer drum; the two components can be assembled to each other. The outer drum can be opened to facilitate assembly therein of the inner drum, and the design is claimed to ensure a constant distance between the primary and secondary coils. The bobbin is configured to use round section wire coils on both drums.
Harwood in U.S. Pat. No. 4,980,664 adds a shroud to a length-divided bobbin to improve safety factors. The bobbin is configured to use round section wire coils on both sides of the divider.
U.S. Pat. No. 5,010,314 to Estrov refers to a low-profile planar transformer for use in off-line switching power supplies. The construction comprises stacking all the planar components required in a sandwich-like laminate, whereafter core pieces are positioned externally. Planar coils and insulation are used, and there are no conventional coils.
The novelty of the transformer bobbin proposed by Shibui et al. in U.S. Pat. No. 5,321,572 comprises mainly a detachably mounted container case. Both parts of the bobbin are wound using conventional coils,
Ichida et al. in U.S. Pat. No. 5,754,086 disclose a transformer unit comprising a main housing accommodating a coil case which in turn encloses a high-voltage coil and magnetic core. The coil case is filled with an insulating compound. Both coils are made of conventional round-section wire.
Planar circuit coils are formed in the stacked device proposed by Weiner in U.S. Pat. No. 5,929,734. This patent was in dispute and was reregistered as belonging to the present assignees. The invention is primarily concerned with terminal pins for connection to printed circuit boards. There are no conventional coils.
Kubomura et al. proposes a side-by-side formation for the coils in U.S. Pat. No. 6,046,663. A novelty is the use of wound steel core around each bobbin coil. The coils themselves are conventional wire wound.
U.S. Pat. No. 6,072,380 to Monroe describes a bobbin-wound current sense transformer wherein a U-shaped staple functions as a one turn primary winding. The secondary winding is of conventional design. The transformer is not height-compact. The single coil may be insufficient for many applications.
Prior art transformer bobbins do not achieve the many advantages offered by planar construction due to the fact that such bobbins are unsuitable where a transformer requires besides the printed circuit windings also a winding with a large number of turns.